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Author Topic: Fuel cooler on Mk. IX - what happened to it?  (Read 4526 times)
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« on: January 30, 2009, 06:31:24 PM »



When the Merlin 63-powered Spitfire Mk IX arrived, it was equipped with fuel cooler installation with port-wing air intake as shown. My understanding is that the cooling installation was required in order to prevent the fuel in the main tak from boiling due to the rapid decrease of air pressure in maximum-power climbs.

I wonder about the physics of the phenomenon, and what further innovation on part of the Rolls-Royce rendered this cooler unnecessary on subsequent variants?

Regards to all,
/Martin
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Antoni
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« Reply #1 on: January 31, 2009, 04:39:40 PM »

Originally developed for the Spitfire Mk VII it was eventually fitted in all versions of the Spitfire with Merlin 61, 63 and 63A engines, including Mk VIII and MK XI.

If the fuel tanks had warmed up in the sun on a bright day, and the aircraft climbed quickly to altitude, the fuel could not lose heat fast enough. Because of the lower outside pressure the fuel would start to boil which could result in a vapour lock in the fuel system. It seems the earliest Mk IXs were not fitted with fuel coolers and their engines would sometimes cut during fast climbs on summer days. BS139 was tested at Boscombe Down in late 1942. Fitted with a fuel cooler the test report stressed no engine cutting at any flight conditions.

Later Merlin versions, Mk 66 and 70, were fitted with the Bendix-Stromberg injection carburettor, which required booster pumps in the base of the fuel tanks to deliver the fuel at higher pressures. The pressure in the fuel system never fell to where boiling would occur so Spitfire LF.IXs and HF.IXs were immune to the problem. Therefore, if a F.IX had its engine replaced with a Merlin 66 or 70 the fuel cooler was no longer necessary and the technical document giving the instructions for such a change starts

The following is the sequence of operations: (i) Remove the fuel cooler installation….
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« Reply #2 on: January 31, 2009, 07:07:07 PM »

That explains it. Thanks, Antoni!
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